Starch/plasticizer composites have been suggested for use in elastomer compositions for various purposes, including tires For example, see U.S. Pat. No. 5,672,639. In U.S. Pat. No. 6,273,163, a first and second coupling agent are sequentially mixed with the rubber composition, thereby substantially decoupling the action of the first coupling agent from the action of the second coupling agent. Various other U.S. patents, for example, U.S. Pat. Nos. 5,403,923, 5,374,671, 5,258,430 and 4,900,361 disclose preparation and use of various starch materials. As pointed in the aforesaid U.S. Pat. No. 5,672,639, starch may represented, for example, as a carbohydrate polymer having repeating units of amylose (anydroglucopyranose units joined by glucosidic bonds) and amylopetin, a branched chain structure, as is well known to those having skill in such art. Typically, starch may be composed of about 25 percent amylose and about 75 percent amylopectin. The Condensed Chemical Dictionary, Ninth Edition (1977), revised by G. G. Hawley, published by Van Nostrand Reinhold Company, Page 813. Starch can be, reportedly, a reserve polysaccharide in plants such as, for example, corn, potatoes, rice and wheat as typical commercial sources.
As discussed in U.S. Pat. No. 5,672,639 a coupling agent in a form of a bis(3-trialkoxysilylalkyl)polysulfide which has an average of from 2 to about 2.6 connecting sulfur atoms in its polysulfidic bridge may be preferred over such polysulfide which has a greater plurality, such as an average of from 3.5 to 4 connecting sulfur atoms because of a lower viscosity buildup, because of less free sulfur generation during the non-productive mixing phase by using the coupling agent with a substantially lower plurality of connecting sulfur atoms.
Accordingly, it is considered herein that, for an organosilane polysulfide which contains an average of only about 2.6 or less, and particularly within a range of about 2 to about 2.6, sulfur atoms in its polysulfidic bridge, the liberation of free sulfur, if any, occurs at a relatively slow rate during a high shear rubber mixing stage, even at a mixing temperature in a range of about 150° C. to about 185° C. depending somewhat upon the overall mixing conditions, including the mixing time itself.
Bis-(3-triethoxysilylpropyl)disulfide, as a variety of organosilane disulfide, is also taught as being useful in silica-containing sulfur-vulcanizable elastomer compositions, even as a high purity form of such disulfide in, for example, U.S. Pat. No. 4,076,550 and German Patent Publication DT 2,360,471.
For further examples of organosilane polysulfides for use as silica couplers, see U.S. Pat. Nos. 4,076,550, 4,704,414 and 3,873,489.
For examples of organosilane disulfides added in a preparatory, non-productive, rubber composition mixing stage, along with a small amount of free sulfur, see U.S. Pat. Nos. 4,076,550, 5,580,919 and 5,674,932.
In practice, sulfur-vulcanized elastomer products are typically prepared by thermomechanically mixing rubber and various ingredients in a sequentially step-wise manner followed by shaping and curing the compounded rubber to form a vulcanized product.
First, for the aforesaid mixing of the rubber and various ingredients, typically exclusive of free sulfur and sulfur vulcanization accelerators, the elastomer(s) and various rubber compounding ingredients are typically blended in at least one, and usually at least two, sequential, preparatory thermomechanical mixing stage(s) in suitable mixers, usually internal rubber mixers. Such preparatory mixing is often referred to as “non-productive mixing”, or “non-productive mixing steps or stages”. Such preparatory mixing may be conducted, for example, at temperatures in a range of about 100° C. to 190° C. and more often in a range of about 140° C. to about 170° C.
Subsequent to such sequential preparatory mix stage(s), free sulfur and sulfur vulcanization accelerators, and possibly one or more additional ingredients, are mixed with the rubber compound, or composition, in a final productive mix stage, typically at a temperature within a range of about 100° C. to about 130° C., which is a lower temperature than the temperatures utilized in the aforesaid preparatory mix stage(s) in order to prevent or retard premature curing of the sulfur-curable rubber, which is sometimes referred to as “scorching”, of the rubber composition.
Such sequential, non-productive, mixing steps, and the subsequent productive mixing step are well known to those in the rubber mixing art.
By thermomechanical mixing, it is meant that the rubber compound, or composition of rubber and rubber compounding ingredients, is mixed in a rubber mixture under high shear conditions where the rubber composition autogeneously heats up, with an accompanying temperature rise, as a result of the mixing primarily due to shear and associated friction within the rubber mixture in the rubber mixer.
Such thermomechanical rubber compound mixing procedure and associated shear and accompanying temperature rise aspects are well known to those having experience in the rubber preparation and mixing art.
It is believed that the prescribed procedure of substantial decoupling of coupling agent reaction with the precipitated silica and reaction with a starch/plasticizer followed by blending carbon black wherewith is novel and inventive in view of past practice.
The term “phr” as used herein, and according to conventional practice, refers to “parts by weight of a respective material per 100 parts by weight of rubber, or elastomer”.
In the description of this invention, the terms “rubber” and “elastomer” if used herein, may be used interchangeably, unless otherwise prescribed. The terms such as “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and “rubber compounding” or “compounding” may be used to refer to the “mixing of such materials”. Such terms are well known to those having skill in the rubber mixing or rubber compounding art.
A reference to the “Tg” of an elastomer, if used herein, refers to a “glass transition temperature” which can be determined by a differential scanning calorimeter at a heating rate of 10° C. per minute.